After the botched landing on Reach which caused the death of four Spartans, Red Team secured the Orbital Defense Generators then split up. Team Delta was ordered to take several Warthogs and the survivors of Charlie Company to secure the Spartan fallback point CASTLE Base. William, Isaac, and Vinh were separated from the team as they fell back to CASTLE Base. They secured a way down to the base's front door, as the elevator had been destroyed but were stuck outside the door until the remainder of Team Alpha used Oly Oly Oxen Free to open the door. Inside they found Doctor Catherine Halsey.

Inside CASTLE Base, Dr. Halsey treated their wounds and sent them on errands for extra armor, prototype weapons, and other items that were there. Soon after, the Covenant started their offensive upon the base and Dr. Halsey was forced to activate Operation: WHITE GLOVE and destroy the base, which forced the team down into the titanium mines below the base. While wandering the mines, Fredric-104 discovered a Forerunner tunnel that led to a massive, twelve story Forerunner cavern which contained a strange Forerunner Crystal.

When the ship jumped into Slipspace, the Forerunner Crystal found on Reach distorted Slipspace. Several other Covenant cruisers were engulfed in the slipspace bubble and a vicious space battle ensued. Shots from the cruisers disappeared and reappeared in random places and destroyed several of the Covenant ships. On board the Ascendant Justice, several rogue units of Elites sabotaged the ships' plasma conduit, forcing the Spartans to perform a dangerous space mission to repair it. When a rogue plasma torpedo hit the dropship, Spartans Anton-044, Li-008 and Petty Officer Second Class Polaski were killed. Their ship then dropped out of the bubble and back into normal space.

The remnants of Team Delta then proceeded to help John-117 in the carrying out of his well planned Operation: First Strike. This involved the Spartans overloading the power core of the Covenant space station Unyielding Hierophant and destroying 500 Covenant capital ships. The operation was going well until the group came under attack by a pack of the ape-like Brutes and Grace-093 was killed. The Master Chief overloaded the core and destroyed the ship and almost all of the 500 ships.

The remaining Spartans were assigned with the defense of Earth when the Covenant attacked. The Spartans were sent to numerous places throughout the course of the battle, including Antarctica, the sea floor and, most recently, to Cuba, to defend one of the last Space Elevators on Earth, the COE, and the cargo of nukes there. The Spartans successfully took the Space Elevator and were soon reassigned by Lord Hood.

Lord Hood reassigned the remaining Spartans to Onyx, to help with the defense of the planet. Soon after arrival, they came under attack by several groups of Sentinels and were forced underground by the attacking Covenant. Along with the help of Kurt-051 and the Spartan-IIIs, Ash, Olivia, Mark, Tom, Lucy, Holly and Dante, they repulsed the Covenant and destroyed three Sentinel production plants before hunkering down at the top of a pyramid. The Spartans set up traps all over the base, but these were soon destroyed by the Grunts. A pair of Hunters made it to the top and would have killed Kelly, had it not been for Holly’s sacrifice. Will then engaged one of the Hunters and killed it in hand-to-hand combat while the others watched in amazement. Will was then killed when the other Hunter shot him point-blank with its Fuel Rod Gun.